1. Field of the Invention
The present invention relates generally to motor control systems and, more particularly, to a pulse width modulation digital motor control system to control both the motor speed and direction of rotation for D.C. or A.C. electric motors.
2. Description of the Background
Prior art servo control systems often either do not provide motor rotation direction control or have problems related to changing the motor rotation direction. The speed of a servo motor and its direction of rotation are conventionally controlled in a linear or analog servo loop as shown in FIG. 1 by controlling the amplitude and polarity of a signal applied to a power transistor included in the loop. However, depending on operation demands, the power transistor may dissipate a lot of power thereby making the system more expensive and less efficient. Inefficiencies in this system are especially large around the null error signal level, i.e., where the motor is at the null between forward and reverse directions. For example, the loss of power for a 12 volt D.C. motor drawing two amperes of current could be approximately 24 watts around null in the power stage of the system shown in FIG. 1. For a servo power control system to operate reliably without failure, the system design should always consider the power dissipation which is a significant factor and also the flexibility of the design in allowing its application to large and small motors. This may be especially important in applications where the available power may be limited.
Motor control has also been provided in non-linear servo loops such as in the servo loop shown in FIG. 2. In this system, the servo loop utilizes an error signal to induce full forward or reverse motor rotation. However, stabilization is difficult because there is no continuous error signal provided at the null position. Instead there is a dead space.
Open loop motor speed control systems have used variations of the time durations of pulses applied to the motor due to their efficient use of power especially useful for battery operated devices. An example is in application to a variable speed drill, screwdriver, or socket driver wherein a mechanical switch is used to control motor direction. The inconvenience in having to mechanically change motor direction is normally acceptable, although it would be desirable for some workers who have only one hand for operation to be able to change motor direction without manually operating a switch.
Moreover, when this type of system is used in a closed loop servo system, the motor must typically be able to reverse automatically, not by a mechanical switch, because the motor could turn forward or reverse many times a second, especially around null and especially in a closed loop system around the null. Therefore, the pulse width variation type of control is not readily adaptable to stable operation of closed loop servo systems.
U.S. Pat. No. 3,942,084, issued Mar. 2, 1976, to Louth, discloses a motor drive and servo systems particularly useful in high quality broadcast video tape recorders. A sine/cosine drive for a brushless DC motor permits high motor efficiency in a system adapted for use in a servo loop. A technique for phase locking a pair of frequency related phase locked control variable signals to a pair of frequency related reference signals, horizontal and vertical sync signals, for example, provides the advantages and precision of closed loop correction at widely variable correction rates. More accurate tape shuttling in a VTR is provided by running a DC motor in a phase locked loop as a synchronous motor and more accurate stopping of the tape is provided by comparing the capstan speed to ground in a closed loop. Improved tape tension control in the head area is provided by a pair of vacuum columns controlled by an error signal derived from the peak-to-peak tension error.
U.S. Pat. No. 4,100,012, issued Jul. 11, 1978, to Meihofer et al, discloses a web splicing apparatus that employs a pair of driven nip rolls which controllably feed web from a running roll into a festoon as web is drawn out of the festoon at a constant rate by a downstream web consuming machine. The nip rolls are driven by a DC motor connected in a closed loop servo system which compares the speed of the web entering the festoon with the web line speed to develop a command signal for the motor. During normal operation, the command signal includes a web velocity trim signal developed by monitoring the position of the festoon dancer relative to a selected reference position so as to minimize tension upsets and to maintain the dancer within its control range. During a splice sequence, the command signal comprises a deceleration ramp having a selected slope to provide controlled deceleration of the web to minimize tension upsets and to permit actuation of the splicing nips prior to actual web stop. After the splice is made, the command signal comprises an acceleration ramp whose slope is automatically adjusted to apply the least necessary tension to the ready web for new roll acceleration consistent with a given splicing speed. Further with this arrangement, the gain of the system is independent of the changing size of the expiring roll.
U.S. Pat. No. 5,334,924, issued Aug. 2, 1994, to Kawada et al, discloses that speed control of an induction motor is effected in digital fashion through use of a computer but without complex processing, and with a computer that need not be large in scale. This is accomplished by processing at least a speed command signal, actual speed signal and torque signal in analog fashion, enabling simplification of an induction motor speed control digital processing section which performs all other control operations in a digital manner. In a speed control network having a closed loop, a frequency-to-voltage converter, adder-subtractor, proportional integrator, polarity determining circuit absolute value circuit and voltage-to-frequency converter are constructed of circuitry operable on the basis of analog values, with all other circuits being constructed of circuitry operable on the basis of digital values.
U.S. Pat. No. 5,729,067, issued Mar. 17, 1998, to Janutka, discloses an improved method and servo control apparatus for controlling the motion of a linear electric motor which in turn generates motion command signals to various apparatus such as a hydraulic steering system. Preferably, the servo control apparatus includes a power supply circuit, a servo amplifier circuit, a pulse width modulation circuit, an H-bridge drive circuit and an inductive position sense circuit. The voltage at a node between coil pairs in the motor is sensed and synchronously demodulated using transmission gates to develop a DC signal representative of armature position from a center location. The signal on a current shunt resistor is synchronously demodulated by transmission gates to generate a signal, the phase of which is determined with respect to the motor drive signal. The phase signal directly indicates whether the armature is off center towards drive coil or drive coil.
U.S. Pat. No. 6,018,200, issued Jan. 25, 2000, to Anderson et al, discloses the throttle of an engine in an engine driven generator system operating subject to a wide and rapidly variable load, as in supplying current to a welder, is operated such that control signals are sent to a throttle actuator for adjusting the engine throttle position in response to load changes. The throttle actuator may be a solenoid pulling against a spring in accordance with the average current through the solenoid coil. In this embodiment, the processor causes pulse width modulated signals to be applied across the solenoid coil with throttle position changes being reflected in changes to the width of the pulses, such changes in the pulse width being delayed for at least the predetermined time since the last preceding adjustment to the throttle. Alternatively, the throttle actuator may be a stepper motor which is stepped by throttle position change signals from a processor which monitors engine speed and generator load to determine whether the throttle should be adjusted and, if so, in which direction and to what extent for optimum response.
U.S. Pat. No. 6,051,943, issued Apr. 18, 2000, to Rabin et al, discloses a motor control system employing a single Hall sensor providing a position feedback signal to a control circuit. The control circuit includes a tach counter circuit, a ramp mode circuit, an interpolation circuit, and a commutation logic circuit. Drive signals are output to the motor windings by the commutation logic circuit. The control state defining the drive signals is advanced on the basis of the estimated rotor position. The estimate of the rotor position is determined by linearly interpolating between Hall signal transitions.
U.S. Pat. No. 6,058,081, issued May 2, 2000, to Schell et al, discloses an optical drive system that includes an objective lens subassembly for directing light from a light source toward an information storage medium. An amount of the directed lighted light is returned from the storage medium. An objective lens is disposed in the objective lens subassembly. A first servomotor moves, during focus capture, the objective lens to a first position, away from the first position toward the storage medium being read while looking for a maximum Quad Sum signal, and back away from the storage medium. An electronic control circuit is connected to the first servomotor. A servo error detector is coupled to the electronic control circuit and disposed in a path of light returning from the information storage medium. The servo error detector is implemented to determine when total light returned from the information storage medium exceeds a predetermined value, to search for a first zero crossing, corresponding to when the Quad Sum signal exceeds a predetermined amplitude, and to indicate to the electronic control circuit to direct close of focus when the Quad Sum signal exceeds the predetermined amplitude.
U.S. Pat. No. 6,064,172, issued May 16, 2000, to Kuznetsov, discloses a winding fault detection system that provides classification and identification of winding faults or winding malfunctions. The fault detection system provides signals to individual electronic switches for segmented primary windings each having an electrical phase and grouped into sub-phases which are individually switch into or out of an excitation supply or isolated through the electronic switching in response to signals from the winding fault detection system. Each primary winding forms an electrical member which includes a stator having a poly-phase winding, and there is a secondary electrical member magnetically coupled with the stator. Each primary has magnetic field sensors which detect phase angle and magnitudes of radial components of air gap flux by magnetic measurement probes between each secondary electrical member and each primary electrical member and derives an electrical signal for a component of air gap flux contributing to electromagnetic torque at each position of each stator's periphery. Additionally, the system instantaneously stores data continuously derived from the magnetic sensors and determines a hierarchy of fault detection schemes.
U.S. Pat. No. 6,069,857, issued May 30, 2000, to Schell et al, discloses an optical disc drive system that is employed in conjunction with a storage medium having a plurality of data sectors each provided with a header and a data storage area. The system includes a data detection device for retrieving stored data from the storage medium and outputting a data signal, an amplifier for providing a variable gain to the data signal and outputting an amplified data signal, a detector that is responsive to the amplified data signal for evaluating a predetermined one of the sectors to ascertain whether the storage area is blank, and an automatic gain control circuit producing a gain control output for controlling the gain of the amplifier. The control circuit has a first mode and a second mode, the first mode being active during retrieval of the header and the second mode being active during retrieval of the data storage area. The system is further provided with a sampling device for sampling the gain control output during retrieval of the stored data in a respective one of the storage areas containing previously stored data. The sampling device outputs results of the sampling, and a fixed gain control circuit is responsive to the results of the sampling for outputting a fixed gain control signal. The fixed gain control signal is applied to the amplifier during evaluation of the predetermined one of the sectors.
It would be desirable to provide a low power dissipation control system without the disadvantages of the systems discussed above, especially at the null between reverse and forward motor directions. Consequently, there remains a long felt need for an improved motor speed and direction control system. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.